1. Field of the Invention
The present invention relates to projection exposure method and apparatus used in forming fine patterns for semiconductor integrated circuits, liquid crystal displays, or the like.
2. Related Background Art
A projection exposure apparatus as called as a stepper is provided with a projection optical system. This projection optical system is incorporated into the exposure apparatus after it is produced in the steps of high-level optical design, strict selection of glass materials, superfine machining of glass materials, and precise assembly and adjustment.
Current steppers actually used in fabrication of semiconductors are so arranged that illumination light of i-line (wavelength 365 nm) from a mercury lamp illuminates a photomask as called as a reticle and that a projection optical system focuses light passing through a circuit pattern on the reticle to form an image thereof on a photosensitive substrate such as a wafer. On the other hand, excimer steppers employing the excimer laser (KrF laser of wavelength 248 nm) are used for evaluation or for research. If a projection optical system for excimer stepper is composed only of refracting lenses, the glass materials usable therefor are limited to quartz or fluorite.
Generally, the resolving power and the depth of focus (D.O.F.) of the projection optical system are important factors to faithfully transfer a fine reticle pattern onto a photosensitive substrate upon exposure with the projection optical system. Some of presently. available projection optical systems for i-line have a numerical aperture (N.A.) of about 0.6. If the wavelength of used illumination light is kept constant, the resolving power increases as the numerical aperture of projection optical system increases. The depth of focus (D.O.F.), however, decreases as the numerical aperture increases. The depth of focus is defined by D.O.F.=xc2x1xcex/(N.A.)2 where xcex is the wavelength of illumination light.
FIG. 1 schematically shows optical paths of image formation in a conventional projection optical system. The projection optical system is composed of a front-group lens system GA and a rear-group lens system GB. The projection optical system of this type is generally both-side telecentric on the reticle R side and on the wafer W side or single-side telecentric only on the wafer side.
There are rays L1, L2, L3, La, Laxe2x80x2, Laxe2x80x3. advancing in various directions from a point A on the pattern surface of reticle (i.e., the object plane of projection optical system), among which the ray L1 is traveling at an angle at which it cannot enter the front-group lens system GA. Also, among the rays entering the front lens system GA, the rays L2, L3 cannot pass through a pupil ep (circular region about the optical axis AX) located on the Fourier transform plane FTP in the projection optical system. The other rays La, Laxe2x80x2, Laxe2x80x3 pass through the pupil ep and then enter the rear-group lens system GB to converge at a point Axe2x80x2 on the surface of wafer. Accordingly, a point image is formed at the point Axe2x80x2 by only the rays passing through the pupil ep in the projection optical system out of the rays leaving the point A on the reticle.
Out of the rays traveling from the point A to the point Axe2x80x2, the ray La passing through the center point CC of pupil is called as a principal ray. If the projection optical system is teledentric on the both sides, the principal ray La travels in parallel with the optical axis AX in the both spaces on the object plane side and on the image plane side.
The above is also the case with rays leaving another point B or C on the reticle, and a point image is thus formed at point Bxe2x80x2 or Cxe2x80x2 only by rays passing through the pupil. Similarly, a ray Lb, Lc travels in parallel with the optical axis AX from the point B, C to enter the lens system GA, becoming a principal ray passing through the center point CC of pupil.
As described, the pupil is in relation of Fourier transform or inverse transform with the pattern surface of reticle or with the surface of wafer so that among the rays from the pattern on the reticle the rays contributing to image formation all superimposedly pass through the pupil.
Discussing the numerical aperture of projection optical system, there are a numerical aperture NAw on the wafer side (on the image plane side) and a numerical aperture NAr on the reticle side (on the object side). The numerical aperture NAw on the wafer side corresponds to an angle xcex8w which the ray Laxe2x80x2 or Laxe2x80x3 passing through the outermost portion of pupil among the image-forming rays toward the point Axe2x80x2 makes with the principal ray La on the wafer. The numerical aperture NAw is expressed as NAw=sin xcex8w. Also, the numerical aperture NAr on the reticle side corresponds to an angle xcex8r which the ray Laxe2x80x2 or Laxe2x80x3 makes with the principal ray La on the reticle side. The numerical aperture NAr is expressed as NAr=sin xcex8r. Further, there is a relation of NAr=Mxc2x7NAw where M is an imaging magnification of projection optical system.
Incidentally, the resolving power can be enhanced by increasing the numerical aperture NAw or NAr. In other words, the resolving power can be increased by making the diameter of pupil larger and further making the effective diameter of lens system GA, GB larger. Nevertheless, the depth of focus (D.O.F.) decreases in proportion to an inverse of square of numerical aperture. Then, even if the projection optical system could be produced with high numerical aperture, the depth of focus would be insufficient, which would be a hindrance in practical use.
When the illumination light is the i-line of wavelength 365 nm and the numerical aperture NAw is 0.6, D.O.F. is about 1 xcexcm (∓0.5 xcexcm) across. As a result, the resolution is poor in a portion where the surface unevenness or curvature exceeds the D.O.F. in a shot area (a square of about 20 mm to 30 mm) on the wafer. As for the system of stepper, the focusing and leveling must be carried out with considerably high precision for each shot area on the wafer, which increases loads on the mechanical system, the electric system and the software (that is, requiring efforts to improve the measurement resolution, the servo control precision, the setting time, and so on).
Applicant proposed a novel projection exposure technology which can provide both a high resolving power and a deep depth of focus, solving such various problems in the projection optical system, as described in Japanese Laid-open Patent Application No. 4-101148 (corresponding to a United States Patent Application, U.S. Ser. No. 847,030 filed on Apr. 15, 1992) or in Japanese Laid-open Patent Application No. 4-225358 (corresponding to a United States Patent Application, U.S. Ser. No. 791,138 filed on Nov. 13, 1991) for example. This exposure technology increases an apparent resolving power and an apparent depth of focus by controlling illumination in a special shape on the reticle without changing the projection optical system, which is called as SHRINC (Super High Resolution by Illumination Control) method.
In the SHRINC method the reticle is illuminated by two or four illumination beams symmetrically inclined with respect to the pitch direction of line-sand-space pattern on the reticle. The zeroth-order-diffracted-light component and one of xc2x1first-order-diffracted-light components, which are emergent from the line-and-space pattern, are let to pass through the pupil ep of projection optical system symmetrically with each other with respect to the center point CC therein. Interference (double beam interference) effect occurs between the zeroth-order-diffracted light and one of the first-order-diffracted light to produce a projection image (i.e., interference fringes) of the line-and-space pattern. Such image formation utilizing the double beam interference can suppress occurrence of wavefront aberration in a defocus state, more than in ordinary perpendicular illumination in conventional systems, increasing the apparent depth of focus.
The SHRINC method can enjoy the desired effect with a periodic pattern formed on the reticle, but has no such effect can be expected for an isolated pattern such as a contact hole. Generally, in case of, an isolated fine pattern, most of diffracted light therefrom is due to the Fraunhofer""s diffraction, so that the diffracted light is not clearly split into the zeroth-order-diffracted light and higher-order-diffracted light in the pupil ep of projection optical system.
Then there is an exposure method for increasing the apparent depth of focus for isolated pattern such as contact hole, proposed for example in U.S. Pat. No. 4,869,999, in which exposure is effected at a plurality of steps for each shot area on the wafer and the wafer is moved by a constant amount along the optical axis between steps. This exposure method is called as FLEX (Focus Latitude Enhancement Exposure) method, which is fully effective to increase the depth of focus for isolated pattern such as contact hole. However, sharpness of resist image obtained after development is inevitably lowered, because the FLEX method requires multiple exposure of contact hole image in a slightly defocused state. This problem of sharpness decrease (profile degradation) can be compensated for by using a resist with high gamma value, a multi layer resist, or a Contrast Enhancement Layer (CEL).
There is an attempt known to increase the depth of focus in projection of contact hole pattern without moving the wafer along the optical axis in exposure process as in the FLEX method, for example the Super-FLEX method as presented in Extended Abstracts 29a-ZC-8, 9 (The Spring Meeting, 1991); The Japan Society of Applied Physics and Related Societies. In the Super-FLEX method, a transparent phase plate is provided in the pupil of projection optical system so that the phase plate gradually changes the complex amplitude transmittance of imaging light from the optical axis toward the periphery thereof. Such arrangement can permit the image focused by the projection optical system to keep sharpness within a certain range along the optical axis about the best focus plane (which is a plane conjugate with the reticle), increasing the depth of focus.
Among the various conventional techniques as described above, the FLEX method and the Super-FLEX method can obtain a sufficient increase in depth of focus for isolated contact hole pattern. It is, however, found that with a plurality of contact hole patterns closely juxtaposed at a certain distance either one of the above methods causes undesirable film reduction in photoresist between the holes and that they cannot be virtually used for close contact hole patterns.
Further, the FLEX method can increase the depth of focus for isolated contact hole pattern on one hand, while it has such a problem:that the exposure dose latitude or margin has to be decreased to maintain the sharpness of synthetic optical image obtained by multiple exposure on the other hand. The FLEX method also has other problems such as difficult application to exposure apparatus of scanning exposure method or a great drop in processing capability.
It is a main object of the present invention to. provide projection exposure method and apparatus which can increase the depth of focus in projection exposure of isolated pattern such as the contact hole.
It is another object of the present invention to provide projection exposure method and apparatus which can increase the depth of focus without any trouble for a plurality of isolated patterns disposed relatively close to each other.
The present invention is applied to a projection exposure apparatus provided with illumination means for illuminating a mask, on which a fine pattern is formed, with exposure illumination light and a projection optical system for receiving emergent light from the pattern on the mask and focus-projecting an image of pattern onto a sensitized substrate. The present invention is characterized in that there is coherence reducing means located on or in the vicinity of a Fourier transform plane (as will be referred to as pupil plane) in an imaging optical path between the mask and the sensitized substrate. The coherence reducing means functions to reduce the coherence between imaging light distributed in a circular region around the optical axis of projection optical system on or in the vicinity of the Fourier transform plane and imaging light distributed in a region outside the circular region.
In the present invention the coherence reducing means makes a part of imaging light which is distributed in a circular or annular region in the pupil plane in projection optical system, incoherent with imaging light which is distributed in the other region than the circular or annular region. As a result, after an imaging beam passes and is diffracted especially through a contact hole pattern in the reticle pattern, it is spatially split into two beams incoherent with each other in the pupil plane then to reach an exposed object such as a wafer. Since the two beams are incoherent with each other also on the wafer, an intensity-synthetic image in respect of quantity of light is obtained from images of contact hole produced by the respective beams.
If there is no phase shift plate provided on the pupil plane of projection optical system, all rays have the same path length from an arbitrary point on reticle to a corresponding image point on wafer in the best focus state irrespective of the ray path in the projection optical system (according to the Fermat""s principle). Accordingly, the amplitude synthesis on wafer is synthesis of beams having no phase difference, so that the intensity of contact hole pattern is enhanced everywhere.
When the wafer is defocused, the above path length differs depending upon the ray path in the projection optical system. Then the above amplitude synthesis results in addition of beams having path difference (phase difference), which causes partial canceling effect to lower the center intensity of contact hole pattern. The thus caused path difference can be approximately expressed by xc2xd (xcex94Fxc2x7sin2xcex8), where xcex8 is an incident angle of an arbitrary ray incident into an image point on the wafer and the reference (=0) is a path length of the ray incident normally to the wafer (principal ray). Here xcex94F represents a defocus amount. Since the maximum of sin xcex8 is the wafer-side numerical aperture NAw of projection optical system, if among diffracted light from the fine hole pattern all light passing through the pupil ep is amplitude-synthesized on the wafer at in ordinary procedure, the maximum path difference is xc2xd (xcex94Fxc2x7NAw2). Assuming that the permissible range of depth of focus is a path difference of xcex/4, the following relation holds.
xc2xd(xcex94Fxc2x7NAw2)=xcex/4
Arranging this equation, xcex94F=xcex/(2NAw2), which coincides with the range of generally called depth of focus. For example, assuming that the exposure illumination light is the i-line (wavelength 0.365 xcexcm) currently used and that the numerical aperture is NAw=0.50, the depth of focus xc2x1xcex94F/2 is xc2x10.73 xcexcm. Therefore, the depth of focus leaves little margin to the process step of about 1 xcexcm on the wafer.
On the other hand, the coherence reducing means CCM is provided on the pupil plane (FTP) of projection optical system, as shown in FIG. 2, in the present invention. An imaging beam. (with principal ray of LLp) diffracted by an isolated pattern Pr formed on the pattern plane of reticle R enters a front-group lens system GA in projection optical system PL and then reaches the Fourier transform plane FTP. On the Fourier transform plane FTP, a part of the beam passes through a circular transmissive portion FA in the central portion in pupil plane ep and the other part of the beam passes through an annular transmissive portion FB while being controlled (changed) into an incoherent state with each other. No interference is thus caused between a beam LFa passing through the circular transmissive portion FA in the coherence reducing means CCM and a beam LFb passing through the peripheral transmissive portion FB. As a result, the beam LFa from the circular transmissive portion FA interferes with itself independent of the beam LFb from the peripheral portion FB, and vice versa, forming an image (intensity distribution) Prxe2x80x2 of hole pattern. Namely, the image Prxe2x80x2 of isolated pattern such as contact hole is obtained in the present invention by simply intensity-adding the image produced on the wafer by the self interference only of the beam LFa and the image produced by the self interference only of the beam LFb.
It is here presumed that the illumination light ILB to the reticle has a constant numerical aperture of sin "PHgr"/2 as in conventional procedure. As for the reticle-side numerical aperture NAr of projection optical system PL, the condition of NAr greater than sin "PHgr"/2 is set.
The principle of image formation in the present invention will be further described referring to FIGS. 3A and 3B.
FIGS. 3A and 3B schematically show the structure of coherence reducing means CCM, a state of an imaging beam forming an image Prxe2x80x2 of contact hole, and the path difference xcex94Z in beam in defocus in relation with each other.
As shown in FIG. 3A, the angle of beam LFa ranges from the normal incidence (principal ray LLp) to the incident angle xcex81 in amplitude synthesis within the beam LFa passing through the central portion. Then, if the defocus amount is xcex94F, the maximum xcex94Z1 of path difference is xcex94Z1=xc2xd (xcex94Fxc2x7sin2xcex81). In each drawing the lowermost graph is depicted with the horizontal axis as sine of incident angle and sin xcex81=NA1. On the other hand, as shown in FIG. 3B, the incident angle range of beam LFb ranges from the incident angle xcex81 to the numerical aperture NAw (sin xcex8w) in amplitude synthesis within the beam LFb passing through the peripheral portion. Then, if the defocus amount is xcex94F, the maximum path length difference xcex94Z2 is obtained as follows:
xcex94Z2=xc2xd(xcex94F)(NAw2xe2x88x92sin2xcex81).
Since the first beam LFa does not interfere with the second beam LFb, deterioration of the image Pr1xe2x80x2 by the interference only of beam LFa or the image Pr2xe2x80x2 by the interference only of beam LFb is caused only by path length difference xcex94Z1, xcex94Z2 in each beam.
For example, if sin xcex81 is set as sin2xcex81=xc2xd (NAw2), that is, if the radius of first transmissive portion FA is set to satisfy the relation of sin xcex81=NAw/{square root over (2+L )}, the maximum path difference xcex94Z1 in the first beam LFa and the maximum path difference xcex94Z2 in the second beam LFb are as follows.
xcex94Z1=xc2xd(xcex94Fxc2x7sin2xcex81)=xc2xc(xcex94Fxc2x7NAw2)
xe2x80x83xcex94Z2=xc2xd(xcex94F)(NAw2xe2x88x92sin2xcex81)=xc2xc(xcex94Fxc2x7NAw2)
Thus, the two incoherent beams LFa, LFb have nearly the same maximum path difference of xc2xc (xcex94Fxc2x7NAw2) with defocus of xcex94F, which is a half of that in conventional procedure. In other words, the maximum path difference is the same even for defocus amount (2xcex94F), which is a double, of that in conventional procedure, as that for defocus amount xcex94F in conventional projection procedure. Consequently, the depth of focus is nearly doubled in image formation of isolated pattern Pr. Such a method for converting an imaging beam into a plurality of beams incoherent with each other in the pupil plane ep of projection optical system PL will be hereinafter called as SFINCS (Spatial Filter for Incoherent Stream) method.
In a preferable embodiment of the present invention, the above coherence reducing means CCM is constituted by polarization control means. PCM for making polarization different between a plurality of imaging beams. The PCM spatially splits an imaging beam passing through a contact hole pattern on reticle while being diffracted, into a plurality of beams incoherent with each other in the pupil plane, which reach the wafer.
In another preferable embodiment of the present invention, the above coherence reducing means CCM is constituted by a path length difference control member for making the path length different from each other between a plurality of imaging beams.
In still another embodiment, the coherence reducing means CCM is constituted by a plurality of space filters alternately arranged in pupil. The plurality of space filters comprise a first space filter for permitting an imaging beam distributed in a region around the optical axis in the pupil of projection optical system to pass therethrough and a second space filter for permitting an imaging beam distributed in the region outside the central region to pass therethrough. Then exposure is effected on the wafer through a first exposure step using the first space filter and a second exposure step using the second space filter.
It is a further object of the present invention to provide a projection exposure apparatus which reduces coherence especially between beams respectively passing through a plurality of contact holes disposed close to each other on the reticle, so as to suppress the xe2x80x9cproximity effectxe2x80x9d which could cause connection of images between the images of plural contact holes.
To achieve this object, the present invention provides a projection exposure apparatus provided with the interference reducing means as described above and having a relatively large "sgr" (ratio between numerical aperture of illumination beam and reticle-side numerical aperture of projection optical system), for example a "sgr" value of not less than 0.5.